Apparatus for feeding, compressing, liquid extraction, washing, chemical treatment and air-drying of sludge, slurries or other wet materials are well known. Such equipment finds particular application in the pulp and paper industry, waste water treatment, mineral processing, agriculture, food processing, fisheries, breweries, wineries, chemical processing, oil and tar sands industry, etc.
Most "in batch" systems, like the plate-and-frame or recessed-plate filter press, perform only filtration by using the feed pressure to force the liquid out through the particulate mass of the material undergoing treatment and the filter medium. Some "in batch" systems, like the diaphragm-press, combine filtration and mechanical expression to produce a dryer cake and shorter cycles; they may also perform washing and air-drying in the same pressure chamber as described in U.S. Pat. No. 4,565,602. These "in batch" systems are best suited for low-consistency slurries containing fine particles. They are not particularly effective at treating pastes, and slurries containing large particles. A drawback of all these "in batch" presses is that the particles start filling the pressure chamber at the periphery, then the filtration area diminishes continuously until the cake completely fills the pressure chamber and can be discharged. Moreover, they have some other serious drawbacks: 1) the yield tends to be low, 2) the filter media has to be washed periodically, 3) the cake handling is intermittent, 4) the odours from the material being processed can't be contained, 5) the press equipment is complicated and cumbersome, and 6) the press equipment is expensive to manufacture, operate and maintain.
Continuous systems have been developed to overcome the problems of the "batch" process equipment. The U-channel rotary press described in U.S. Pat. No. 4,534,868 is best suited for pastes and fibrous solid-liquid material, but it is ineffective in handling low-consistency slurries containing fine particles. Conversely, the rotary press described in U.S. Pat. No. 5,344,575 is best suited for low-consistency slurries containing large particles, but it is ineffective in treating slurries containing fine particles, pastes, and fibrous solid-liquid materials. Moreover, both rotary presses have some other serious drawbacks: 1) they can not perform washing and air-drying, 2) each processing channel of the equipment has to be isolated from the others, totally or partially, by complicated sealing systems, 3) the pressure and the outlet consistency of the material are poorly controlled by squeezing the cake with restricting plates, 4) a slip-stick movement of the cake may occur, 5) the pressure has to be controlled separately for each processing channel, 6) the press design is difficult to optimize since the channel width determines both the cake thickness and the pressure gradient.
The method and apparatus described in U.S. Pat. No. 4,986,881 is a modification of the rotary press described in U.S. Pat. No. 4,534,868 intended to enable it to perform washing and chemical treatment on solid-liquid material. However, it still has all the other drawbacks of the two above mentioned rotary presses.
The compression feeder described in U.S. Pat. No. 5,213,686 is a further modification of the U-channel rotary press described in U.S. Pat. No. 4,534,868 intended to enable the feeder to perform compaction into a plug of such compressible materials as wood chips, high consistency paper pulp, wax and fibrous waste, etc. However, this feeder still suffers from some of the drawbacks of the prior art, namely, the pressure is poorly controlled by squeezing the cake with restricting plates and a slip-stick movement of the cake may occur during processing.